When she was a young physician, Dr. Martha Gulati noticed that many of her mentors were prescribing vitamin E and folic acid to patients. Preliminary studies in the early 1990s had linked both supplements to a lower risk of heart disease.

She urged her father to pop the pills as well. But a few years later, Gulati, now chief of cardiology for the University of Arizona College of Medicine-Phoenix, found herself reversing course after rigorous clinical trials found neither supplement did anything to protect the heart. Even worse, studies linked high-dose vitamin E to a higher risk of heart failure, prostate cancer and death from any cause. “‘You might want to stop taking [these],’ ” she told her father.

More than half of Americans take vitamin supplements, including 68 percent of those 65 and older, a 2013 Gallup poll said. Among older adults, 29 percent take four or more supplements, according to a Journal of Nutrition study.

Often, preliminary studies fuel exuberance about a dietary supplement, leading millions of people to buy in to the trend. Many never stop. They continue even though more rigorous studies — which can take many years to complete -— hardly ever find that vitamins prevent disease, and in some cases cause harm. “The enthusiasm does tend to outpace the evidence,” said Dr. JoAnn Manson, chief of preventive medicine at Boston’s Brigham and Women’s Hospital.

There’s no conclusive evidence that dietary supplements prevent chronic disease in the average American, Manson said. And while a handful of vitamin and mineral studies have had positive results, those findings haven’t been strong enough to generally recommend supplements, she said.

The National Institutes of Health has spent more than $2.4 billion since 1999 studying vitamins and minerals. Yet for “all the research we’ve done, we don’t have much to show for it,” said Dr. Barnett Kramer, director of cancer prevention at the National Cancer Institute.

Part of the problem, Kramer said, could be that much nutrition research has been based on faulty assumptions, including the notion that people need more vitamins and minerals than a typical diet provides; that megadoses are safe; and that the benefits of vegetables can be boiled down into a pill.

Vitamin-rich foods can cure diseases related to vitamin deficiency. Oranges and limes prevent scurvy. And research has long shown that populations that eat a lot of fruits and vegetables tend to be healthier than others.

But when researchers tried to deliver the key ingredients of a healthy diet in a capsule, Kramer said, those efforts nearly always failed. It’s possible that the chemicals in the fruits and vegetables on your plate work together in ways that scientists don’t fully understand, said Marjorie McCullough, strategic director of nutritional epidemiology for the American Cancer Society.

More important, perhaps, is that most Americans get plenty of the essentials. Although the Western diet has a lot of problems — too much sodium, sugar, saturated fat and calories, in general — it’s not short on vitamins, said Alice Lichtenstein, a professor at the Friedman School of Nutrition Science and Policy at Tufts University.

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